1. Field of the Invention
The present invention relates to a circuit arrangement for generating motor characteristic curves, in particular for stabilizing the motor pull-out torque of a two phase synchronous motor by using a series-connected element, wherein the series-connected element is connected between a first line conductor formed as a neutral conductor and the common phase of the synchronous motor, and a second line conductor implemented as a conductor is connected with a phase of the synchronous motor.
2. Description of the Related Art
Synchronous motors are reliable actuators for many price-sensitive applications in the field of automotive and office communication, medical technology, tool making, consumer electronics, building equipment or measurement technology.
In the building equipment field, synchronous motors are employed, for example, as valve actuators for heating equipment. By combining a relatively small synchronous motor with the gear, very high actuating forces can be produced due to the gear reduction.
Because synchronous motors operate directly on the AC voltage grid, there is typically no need to change the operating voltage. The second phase for synchronous motors configured for two-phase operation is formed from the line phase by a phase capacitor CM. Commonly used is a parallel connection, whereby a common phase is formed.
Operation of synchronous motors with different line voltages typically requires as a series-connected element an ohmic resistor RV or a capacitive resistance CV.
The rotation speed of the synchronous motor is determined by its design, in particular by the number of poles 2p of a stator, and the frequency f of the power grid. The rotation speed does not depend on the torque.
The synchronous motor rotates with a synchronous rotation speed and can hence be loaded from idle speed to a maximum torque without changing its rotation speed. This maximum torque is referred to as pull-out torque MK. It represents a stability limit for the synchronous motor, because at higher applied loads the synchronous motor runs unstable and comes to a stop. It is disadvantageous for the practical applications that the pull-out torque MK is voltage-dependent. There exists a direct proportionality MK=f(U) between the pull-out torque MK and the supply voltage U, so that precisely limiting the torque of the synchronous motor becomes very difficult due to line voltage tolerances.
Conventional applications of synchronous motors which require torque limitation include a combination of synchronous motor/synchronous gear motor and a spring mechanism, whereby the spring mechanism activates a micro-switch at a specified motor load torque which is less than the pull-out torque MK, wherein the micro-switch then switches the synchronous motor off.
In another alternative embodiment for intentionally switching a synchronous motor off when a specified motor load torque is reached, a coupling/clutch with permanent magnets for force transmission or a friction coupling is provided. In addition to the advantage provided by this mechanical switch-off, the same mechanism can be used in operation for switching the motor off during overload as for switching the motor off at the end of the control range of a synchronous motor. However, a not insignificant disadvantage should be noted, for example the need to employ additional mechanical elements, mechanical wear, reduced service life, the necessity to provide one or more disconnecting elements, or adjustment or control of the switch-off characteristic during production.
A circuit arrangement for switching synchronous motors off is known from WO 02/095926 A1 and DE 200 08 483 U1. By using only a single AC voltage switching element, an inexpensive and reliable implementation of the motor turn-off is achieved when the pull-out torque is exceeded. However, the maximal torque at the time of blocking is here also strongly dependent on the line voltage.
In summary, it can be stated that conventionally thyristors, triacs, photo-triacs, relays and micro-switches are used for limiting the torque of actuators through final disconnection of the supply voltage.
DE 19533076 A1 also describes a control circuit for synchronous motors, whereby asynchronous startup of the motor is realized with a triac.
When using thyristors or triacs, the control circuit is frequently combined with a phase angle control. In this way, the effective AC voltage can de effectively and intentionally reduced and/or controlled. A typical example for an application is a light dimmer. However, employing a phase angle control is disadvantageous for controlling the pull-out torque MK of synchronous motors, because the higher harmonics interfere with the synchronism of the synchronous motor and thereby produce additional noise. Additional noise emission can be eliminated and the proper operation of the adequately dimensioned phase capacitor can be maintained only if the voltage curve remains approximately sinusoidal.
The conventional circuit arrangement described in DE 3509451 A1 uses a pulsed switch operating with a high clock rate for solving the disadvantages of a phase angle control, such as the strong humming of motors. This pulsed switch is controlled by pulse width modulation (PWM), thereby maintaining a sinusoidal supply current. The pulsed switch formed as transistor operates always with very short switching times, preferably with frequencies above the audible range. The high efficiency of this pulsed circuit and the low thermal load of the controller are achieved at the “expense” of a rather complex bypass circuit, or free-wheeling circuit, which is expensive to implement. This bypass circuit must be synchronized with the pulsed circuit to prevent high peak switching voltages that could destroy components. The solution is unsuitable for cost reasons when using claw pole motors with rated powers of several watts. Compared to the general excepted state of the art, there remains the question if employing a classic frequency converter AC/DC-DC/AC would not have the same cost, while providing additional advantages in practical applications. A particular disadvantage of the solution are the interference emissions, both on the line and radiated, which are generated by the fast switching flanks of the pulsed switch. It has been observed that such pulsed power switches require additional costly protective measures regarding electromagnetic compatibility.